Coupling between electronic and structural degrees of freedom in the triangular lattice conductor Na_xCoO₂

The ambient temperature crystal structures of compounds in the Na _xCoO₂ family, for 0.34<x≤1.0, determined by powder neutron diffraction, are reported. The structures consist of triangular CoO ₂ layers with Na ions distributed in intervening charge reservoir layers. The shapes of the CoO₆ octahedra that make up the CoO ₂ layers are found to be critically dependent on the electron count and on the distribution of the Na ions in the intervening layers, where two types of Na sites are available. Correlation of the shapes of cobalt-oxygen octahedra, the Na ion positions, and the electronic phase diagram in Na _xCoO₂ is made, providing an example of how structural and electronic degrees of freedom can be coupled in electrically conducting triangular lattice systems.